Development of an atom-cluster generator for an ultra-high voltage electron microscope

当固体的尺寸减小到几纳米时，其物理和化学性质与块状固体相比发生显著变化。此后，这种超细颗粒被称为原子簇或简单的簇。然而，对原子团簇中的扩散性质的研究还很缺乏。基于这一前提，在本工作中，原子团簇发生器的UHVEM的开发，并与它的使用，在原子团簇的合金化行为进行了检查。发电机由两个螺旋形灯丝制成的钨。该发生器设置在3 MV高压静电发生器的样品室中。细丝和原子簇的支撑膜（基板）之间的距离为约100 mm。非晶碳膜用作支撑膜。利用该发生器，首先在支撑碳膜上沉积了尺寸为~ 3 nm的金簇。然后在相同的膜上形成铜簇。通过透射电子显微镜原位研究了与铜团簇沉积相关的团簇形态和晶格常数的变化。结果表明，在沉积铜的快速溶解成金团簇已经发生。所观察到的铜在金团簇中的扩散率比在大块金中快约9个数量级。研究还表明，金团簇的重心在合金化过程中没有明显的平移运动。这些观察清楚地表明，沉积的铜直接或迁移到衬底上后，撞击并连接到金团簇，并迅速溶解到金团簇的内部，形成均匀混合的Au-Cu合金团簇。这种溶解已经发生，没有可见的平移运动的支持膜上的个别集群。

When the size of a solid decreases to as small as several nm, its physical and chemical properties change remarkably as compared to those of a bulk solid. Hereafter such ultrafine particles are designated as atom clusters or simply clusters. However, studies on the diffusional properties in atom clusters are lacking. Based on this premise, in the present work, an atom-cluster generator for UHVEM was developed, and with the use of it, alloying behaviour in atom clusters was examined. The generator consisted of two spiral-shaped filaments made of tungsten. The generator was set in the specimen chamber of a 3MV HVEN. The distance between the filaments and a supporting film (substrate) for atom clusters was approximately 100 mm. An amorphous cabon film was used as the supporting film. With the use of the generator gold clusters of -3nm in size were first deposited on the supporting carbon film. Copper clusters were then formed on the same film. Change in the morphology and lattice constant of the clusters associated with the copper cluster deposition were studied in situ by transmission electron microscopy. It is revealed that upon the deposition of copper a rapid dissolution of copper into gold clusters has taken place. The observed copper diffusivity in gold clusters is approximately 9 orders of magnitude faster than that in bulk gold. It is also revealed that the center of gravity of the gold clusters has shown no visible translational motion during the alloying. These observations clearly indicate that deposited copper has either directly or after migration on the substrate, impinged upon and joined with gold clusters and has dissolved quickly into the interior of the gold clusters to form homogeneously mixed Au-Cu alloy clusters. This dissolution has taken place without visible translational motionof individual clusters on the supporting film.